Formación profesional

1.0 Introduction 2.0 Objectives 3.0 Main Content

3.1 Quality characteristics of Milk 3.1.1 Flavour

60 3.1.2 Hygiene

3.1.2.1 Total Bacteria Count 3.1.2.2 Somatic Cell Count 4.0 Conclusion

5.0 Summary

6.0 Tutor-Marked Assignment 7.0 References/Further Readings

1.0 INTRODUCTION

In Unit 1 you learnt the importance of milk as regards natural essentiality for survival, composition and nutritional value. However, all these attributes will be of no value if the milk available for use smells bad, tastes bad or can cause disease. In developed countries where milk production and quality standards have been established, producers may be penalized for poor quality milk and/or obtain very low value or payments for it. Therefore, it is important that factors that can negatively affect the value of milk be identified and reduced as much as possible or alleviated.The key to producing quality milk is to have correct information, make the right decisions and carry out the recommended actions correctly. This is the focus of this unit.

2.0 OBJECTIVES

At the end of this unit you should be able to:

• describe the quality characteristics of good quality milk

• identify the differences between good and poor quality milk

• distinguish between total bacteria count and somatic cell count

3.0 MAIN CONTENT

3.1 Quality Characteristics of Milk

Milk that has been harvested from animals and stored at appropriate cold temperatures in tanks is transported to processing centres where they are tested for quality before processed. The milk is checked for protein, fat, flavour, total bacteria count, somatic cell count, and residues. The protein and fat contents are checked against standard milk values to accept or reject the milk. Other attributes checked are discussed in the following sections.

3.1.1 Flavour

Milk is a yellowish-white non-transparent liquid. Fresh milk has a pleasant soft and sweet taste and carries hardly any smell. Consumer acceptance of milk is greatly affected by its flavour. There are several factors which may produce off-flavours

61 and/or odours in milk. Some of the more common causes of flavour and odour problems are:

• Strong flavored feedstuffs such as poor quality silage or feed

• Strong smelling plants, like wild onion or garlic

• High acidity flavours and oxidized flavours, from contact with copper or exposure to sunlight; and flavours from the use of chlorine, fly sprays, medications, etc.

• Rancid flavours: These are caused by excessive agitation of milk during collection and/or transport. Damage of the fat globules in the milk results in the presence of free fatty acids.

• Cow-barn flavors from dung, etc. These are found when milk is obtained from a dirty or poorly ventilated environment or from improperly cleaned milking equipment.

3.1.2 Hygiene

Milk, when it emerges from a healthy udder (the mammary gland of the female animal, which has teats where milk comes out from) contains only a very few bacteria.

However, milk is a perishable product. It is an ideal medium for micro-organisms and as it is a liquid, it is very easily contaminated and invaded by bacteria. Almost all bacteria in milk originate from the air, dirt, dung, hairs, dust, dirty equipment, operators, and other extraneous substances. In other words, milk is mainly contaminated with bacteria during milking.

The major group of bacteria in milk is the lactic acid bacteria, which grow and produce lactic acid rapidly when milk is kept at ambient temperature and it becomes sour. How soon the milk turns sour depends on the degree of contamination and on the temperature of the milk. Therefore, proper cleaning and sanitizing procedures are essential to control the quality of milk. Cooling milk to a temperature of 4ºC as soon as the animal is milked (the first 2 – 3 hours) makes the bacteria inactive and prevents them to grow and produce the lactic acid.

3.1.2.1 Total Bacteria Count

The total population of bacteria in milk is termed Total Bacteria Count (TBC) and very good raw milk should contain only 500 to 1,000 bacteria per ml. The normal total bacteria count after milking is up to 50,000 per ml and many processors may not be willing to accept raw milk with a higher value. However, counts may reach several millions bacteria per ml. This indicates a very poor hygienic standard during milking

62 and the handling of the milk or milk from an animal with disease such as mastitis.

Mastitis is a disease of the udder. This disease arises when bacteria enter the udder and establish an infection. The disease is caused by Streptococcus and Staphylococcus spp bacteria leading to clotting and discolouration of the milk, reddening, heat, pain, swelling and hardening of the udder.

Hygienic milk only originates from mastitis free and healthy animals. Cows suffering from a disease may secrete the pathogenic bacteria, which cause their disease, in the milk they produce. Consumption of raw milk therefore might be dangerous to the consumer. Some of these diseases, like tuberculosis, brucellosis and anthrax, can be transmitted to the consumer. Whatever the milk is used for during processing, the hygienic standard of the produced milk at farm-level forms the basis of the quality of the ultimate milk products.

3.1.2.2 Somatic Cell Count

Apart from TBC, another measure of the quality of raw milk is the Somatic Cell Count (SCC), which is a direct indication of infection as there is a very high correlation between mastitis and bacterial infection. Somatic cells consist almost totally (98%) of white blood cells. When bacteria exist in the environment close to the teat end, the first line of defence against bacterial infection is the teat canal. If the bacteria succeed in entering the udder, then the second line of defence comes into play and inflammation occurs, i.e. white blood cells or somatic cells. These somatic cells try to destroy the bacteria and prevent it from infecting and damaging the udder tissue.

Thus, these somatic cells (SCC) are the first reliable indication of mastitis infection. A certain level of somatic cells are always present in milk, as a protection for the cow against mastitis infection. Most cows that are free of mastitis and have no previous infections would be expected to have an SCC of less than 100,000 cells/ml, with many below 50,000 cells/ml. It is widely accepted that individual cow SCC greater than 150,000 cells/ml or individual heifer SCC greater than 120,000 cells/ml indicates presence of infection.

Milk with a high somatic cell concentration can be harmful to human health and contains less protein. In addition milk with a high cell count generally contains an increased amount of enzymes, which have effect on the quality of the protein and the fat in milk. The presence of these enzymes in milk increases the potential for off-flavours and odours. Because the somatic cell content of raw milk is important for the shelf-life, flavour and the quantity of products obtained, milk processors strive to obtain raw milk of the highest hygienic quality from their producers. .

63 3.1.2.3 Residues in Milk

Good quality milk must be free of various residues that may be deposited or remain in milk along the pre-processing handling of raw milk. Such residues are antibiotics, disinfectants, iodine, trichloromethane (TCM), added water, and sediments.

Milk must be free from antibiotics and traces are not acceptable. All milk supplies are tested for antibiotic residues. Antibiotic residues result from the milk collected from animals treated for various infections, which were not completely disposed. Milk from animals being treated must not be processed or consumed and the proper safe period after treatment must be strictly observed.

Disinfectants and TCM contamination result from improper washing or rinsing of equipment used in the pre-processing of raw milk. The most effective disinfectants have chlorine as the active component but chlorine can form TCM if it comes in contact with organic matter such as milk remaining in processing equipment pipes.

Very low levels or zero levels of TCM are tolerated.

Iodine contamination results from the feed given to animals. In spite of the fact that iodine is very important to human physiology, an excessive level is not healthy.

Therefore, proper monitoring of iodine levels in feed is required.

Water is the most abundant component of milk. However, excess water above acceptable levels is not proper because the consumer is paying for milk and not water.

Excess water could arise from improper draining of water from pipes after washing or switching to automatic cleaning too early.

Physical cleaning of teats before milking is essential for lowering sediment in milk.

Sediment in milk is generally due to poor pre-milking hygiene procedures that allow soil and other materials to enter the milking system. Proper environmental conditions are important in order to maintain cow cleanliness and to reduce soil on animals so that premilking hygiene procedures can be effective. Sediment in milk is measured by filtering the milk through a fine filter and visually examining it. High sediment levels in milk are associated with dirt and increased potential for bacterial contamination, thus adversely influencing milk quality.

SELF ASSESSMENT EXERCISE 4.2

List items considered in determining the quality of milk 4.0 CONCLUSION

The importance of milk as a valuable food source will be lost if it is unacceptable to the end users. Improving milk supply in any economy is not as important as

64 guaranteeing the quality in order to satisfy the objective of nourishing the most needy in the population.

5.0 SUMMARY

In this unit, you have learnt:

• the quality characteristics of good quality milk

• to distinguish between good and poor quality milk

• factors affecting hygienic quality of milk 6.0 TUTOR-MARKED ASSIGNMENT

Write briefly on the significance of somatic cell count, total bacteria count and residues in milk

7.0 REFERENCES/FURTHER READINGS

Early R. (1998). The Technology of Dairy Products. (2nd ed.). London: Blackie Academic and Professional

Robinson R. K. (2002). Dairy Microbiology Handbook. (3rd ed.) New York:

Wiley-Interscience, A John Wiley & Sons, Inc., Publication Tetra Pak Processing

Systems (1995) Dairy Processing Handbook. 1995. Sweden:

Tetra Pak Processing Systems AB

Wehr, H. M., and Frank, J. H. (2004). Standard Methods for the Examination of Dairy Products. (17th Ed.) Washington, DC: American Public Health Association

UNIT 3 MILK PROCESSING AND STORAGE